GPS based search and rescue system

ABSTRACT

A GPS based search and rescue system utilizes an airborne interrogation unit for extending the range of a second interrogation unit. The airborne unit relays communication between a survival radio and the second interrogation unit. The second interrogation unit is typically a ground based unit and is arranged to provide GPS correction information to the survival radio. The normal line of sight communication between a survival radio and an interrogation unit is extended to an over the horizon communication path by utilizing the airborne interrogation unit as a relay for communications.

The present application is a continuation in part of prior U.S.application No. 08/299,029, filed Aug. 31, 1994 now U.S. Pat. No.5,748,147; which is a continuation in part of U.S. application No.08/103,177, filed Aug. 6, 1993 now abandoned; which is a continuation ofU.S. application No. 07/845,903, filed Mar. 4, 1992 now abandoned; whichis a continuation of U.S. application No. 07/845,903, filed Mar. 4, 1992now abandoned; the disclosures of all of which are hereby incorporatedby reference, and priority thereto for common subject matter is herebyclaimed.

FIELD OF THE INVENTION

This invention pertains to radios, in general, and to search and rescueradio systems, in particular.

BACKGROUND OF THE INVENTION

When a search and rescue operation is required, as for a downed airplaneor helicopter pilot for example, search and rescue radio (SAR) systeminterrogators are typically employed. Such search and rescue radiosystems include a small portable radio which is in the possession of thedowned pilot. Such systems may further include an interrogation unitwhich is typically carried in the rescue aircraft (helicopter or plane).

Important information required by the search and rescue aircraftincludes correct identity of the survival radio. Identification isimportant to avoid, for example, decoy signals or traps by hostileforces. Other important information is the location of the radio andassociated downed pilot.

The Global Positioning System (GPS) is particularly useful fordetermining position. Survival radios have been known to have GPSreceivers capable of self determination of position.

Search and rescue interrogation units typically include a GPS receiver,portable computer and a separate transceiver. Communication betweeninterrogation units and survival radios is linked to Line of Sight (LOS)communication links. However, in many cases, it would be desirable toextend the LOS communication link to communication links that couldextend over the horizon (OTH).

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from a reading of the followingdetailed description taken in conjunction with the drawing in which likereference designators are used to designate like elements and in which:

FIG. 1 is a block diagram of a SAR system in accordance with theprinciples of the invention;

FIG. 2 is a block diagram of an interrogation unit in accordance withthe principles of the invention;

FIG. 3 is a block diagram of a SAR radio unit in accordance with theprinciples of the invention;

FIG. 4 illustrates the data format utilized in the system of FIG. 1;

FIG. 5 illustrates in detail a data format for first packet messagestransmitted from the SAR radio of FIG. 3;

FIG. 6 illustrates a data format for second packet messages transmittedfrom the SAR radio of FIG. 3;

FIG. 7 through 10 illustrate message formats.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A search and rescue system in accordance with the principles of theinvention is depicted on FIG. 1. An airborne interrogation unit 1 isoperated aboard the helicopter 2 by one of the occupants of thehelicopter 2. The interrogation unit 1 transmits signals to interrogateany survival radio with GPS capability such as the PRC-112 type survivalradios 3, 4 and 5. The helicopter 2 based interrogation unit 1 is asubstantially Line of Sight (LOS) radio system and communicates to theSAR survival radios in the LOS paths 101, 102, 103.

Each survival radio 3, 4 and 5 has a unique identification number whichthe interrogation unit 1 is programmed to interrogate. Each survivalradio 3, 4, 5 can act as a transponder supplying ranging andidentification information in response to interrogation signals. Eachcan also perform as an emergency beacon by transmitting an emergencyswept tone beacon signal. Each can also provide substantially Line ofSight (LOS) voice communication.

Survival radios 3, 4 and 5 are typically in the possession of a downedpilot or other aircraft personnel. When searching for a downed pilot,the interrogation unit 1 is triggered and a GPS compatible message isgenerated. The interrogation unit 1 is connected to the helicopteravionics transceiver. The interrogation unit may include an integraltransceiver unit of conventional design rather than using the helicoptertransceiver. The helicopter transceiver radio transmits interrogationmessages to the survival radio 3 and receives response message from thesurvival radio 3. When the interrogation unit 1 receives a responsesignal from survival radio 3, the response will be a message including asurvival radio identification number, GPS position information of theposition of the survival radio 3 and status information is obtained.From this information the interrogation unit 1 can quickly determine therange and bearing of the survival radio 3 from the helicopter 2 once theaircraft's position is known.

The interrogation unit 1 plugs into the helicopter 2 intercom/radiosystem which is not shown but may be any conventional radio system.Interrogation unit 1 includes a processor which may be in a personalcomputer coupled to a radio via a modem. The interrogation unit 1 iscapable of transmitting and receiving GPS radio messages includinglocation and messaging information of the survival radio 3, 4, 5 beingsought by the search.

As described above, the interrogation unit 1 communicates with thesurvival radios 3, 4 and 5 which have embedded GPS capability. GPScapability means the ability to self determine position through the useof the GPS constellation of satellites. The Global Positioning System(GPS) may be used to determine the position of a GPS receiver on or nearthe surface of the earth from signals received from a constellation ofsatellites. The orbits of the GPS satellites are arranged in multipleplanes in order that signals can be received from at least foursatellites at any position on earth. More typically, signals arereceived from six or eight satellites at most places on the earth'ssurface.

Orbits of GPS satellites are determined with accuracy from fixed groundstations and are relayed to the spacecraft. The latitude, longitude andaltitude or any point close to the surface of the earth can becalculated from the times of propagation of the electromagnetic signalsfrom four or more of the satellites. A measured range, referred to as a"pseudorange", is determined between the GPS receiver and the satellitesbased upon these propagation times. The measured range is referred to aspseudorange because there is typically a time offset between timingclocks on the satellites and a clock within the GPS receiver. Todetermine a three dimensional position, at least four satellite signalsare needed to solve for the four unknowns represented by the time offsetand the three dimensional position.

The nature of the signals transmitted from the GPS satellites is wellknown from the literature. Each GPS satellite transmits two spreadspectrum, L-band carrier signals, referred to as L1 and L2 signals. Twosignals are needed if it is desired to eliminate any error that arisesdue to refraction of the transmitted signals by the ionosphere. The L1signal from each GPS satellite is Binary Phase Shift Keyed (BPSK)modulated by two pseudorandom codes in phase quadrature. A pseudorandomcode sequence is a series of numbers that are random in the sense thatknowledge of which numbers have been already received does not provideassistance in predicting the next received number. Using a binarypseudorandom code to modulate the phase of a carrier signal produces asuppressed carrier spread spectrum signal. The L2 signal from eachsatellite is BPSK modulated by only one of the pseudorandom codes. Useof the pseudorandom codes allows use of a plurality of GPS satellitesignals for determining a receiver's position and for providingnavigation information. A signal transmitted by a particular GPSsatellite is selected by generating and matching, or correlating, thepseudorandom code for that particular satellite.

Some of the pseudorandom codes are known and are generated or stored inGPS receivers. Other pseudorandom codes are not publicly known.

A first known pseudorandom code for each GPS satellite is referred to asa "clear acquisition" or C/A code. The C/A code is intended tofacilitate rapid satellite signal acquisition and is a relatively short,coarse grained code. The C/A code for any GPS satellite has a relativelyshort length before it repeats. A second known pseudorandom code foreach GPS satellite is referred to as a "precision" or P-code. The P-codeis a relatively long, fine grained code. The full P-code has a length of259 days, with each satellite transmitting a unique portion of the fullP-code. The portion of P-code used for a given GPS satellite has alength of precisely one week before the portion repeats.

In GPS receivers, signals corresponding to the known P-code and C/A-codemay be generated in the same manner as in the satellites. The L1 and L2signals from a given satellite are demodulated by aligning the phases,i.e., adjusting the timing of the locally generated codes with thosereceived from the satellites. To achieve phase alignment, the locallygenerated code replicas are correlated with the received satellitesignals until the resultant output signal reaches a peak. Because thetime at which each particular bit of the pseudorandom sequence istransmitted from the satellite is defined, the time of receipt of aparticular bit can be used as a measure of the range to the satellite.Because the C/A and P-codes are unique to each GPS satellite, a specificsatellite may be identified based on the results of the correlationsbetween the received GPS signals and the locally generated C/A andP-code replicas. Methods for generating the C/A code and P-code are setforth in various publicly available publications.

The C/A code component of the L1 signal is provided for commercial use.Various techniques have been developed to replicate the C/A code in GPSreceivers. As a consequence of the repetition of the C/A-codeapproximately once every millisecond, correlation at the GPS receivermay be performed in the absence of precise knowledge of the time oftransmission of each C/A code bit. Acquisition of the P-code isgenerally acquired by first locking on to the C/A-code. Once theC/A-code has been acquired, the C/A-code modulated carrier component ofthe L1 signal carrier alone may allow for satisfactory measurements.However, when high resolution measurements are desired to be madequickly, the L2 carrier signal must also be used. The unknownatmospheric delay of the L1 and L2 carriers may be eliminated when bothof the L1 and 12 carriers are used.

The GPS signals are intended to be recovered by correlating eachincoming signal with a locally generated replica of each code, both theP-code and the C/A-code. The result of such correlation is that thecarrier in the GPS signals is totally suppressed when the modulatingsignal is the pseudorange code sequence like the P-code or the C/A-code.The locally generated code is adjusted in timing to provide an optimumcorrelation with the incoming signal. The correlation output is then asingle narrowband peak centered at the carrier frequency. The carrierrecovered by correlation provides the best available signal-to-noiseratio.

For various reasons, it is not desirable to utilize the P-codecapability in survival radios or in the airborne interrogation units.These apparatus utilize the line account C/A code.

Absolute GPS position accuracy is improved in a system in accordancewith the invention utilizing the C/A code by the use of GPS differentialcorrection or pseudo range data sent to the survival radios 3, 4 and 5from a ground based interrogation unit 10. The correction data is basedupon an accurate surveyed location of the ground based interrogationunit 10. For SAR applications, the accuracy of the relative positionbetween an interrogation unit disposed in a SAR rescue vehicle and thesurvival radio unit 3, 4 or 5 is equivalent to that afforded bydifferential correction data without the need of a surveyed fixed groundbased interrogator.

The range of the system is extended to an over the horizon (OTH) systembetween the survival radio units 3, 4 and 5 and the ground basedinterrogation unit 10 by utilizing the interrogation unit 1 aboard thehelicopter 2 to relay data between the survival radio units 3, 4 and 5and the ground based interrogation unit 10. A polling protocol isutilized having a unique identification code for each communicationelement. Burst data transmission protocol with bit interleaving, forwarderror correction and encryption in order to maximize the communicationrange of the SAR system and to minimize unauthorized reception andutilization of the transmitted information.

In the system of the illustrative embodiment, an identified number fieldis included in each message transmitted from a survival radio. Thatfield can provide for up to 65,535 survival radio units such as units 3,4 and 5 on one frequency. This results from the use of 65,536 uniqueidentification numbers being allowable in the identification numberfield. One of the identification numbers is reserved for theinterrogation unit 1. Information is passed to and from the survivalradios 3, 4 and 5 to the ground based unit 10 through either a directLOS UHF radio link 104, or through an OTH UHF radio link comprising LOSlink 105 and LOS link 101 by relaying through interrogation unit 1aboard the airborne platform 2. The SAR interrogation unit 1 can operateautonomous of the ground based interrogation unit 10 and can directlyinitiate LOS interrogations of the survival radio units 3, 4 and 5. Thislater mode of operation is useful during the terminal portion of a SARoperation. Transmission and reception of position and messaging databetween the survival radio units 3, 4 and 5 and the interrogation unit 1is facilitated by the use of a digital modem utilizing any one of anumber of modulation formats such as multilevel FSK, PSK, BPSK, MSK orGSK.

Turning now to FIG. 2, a block diagram of the interrogation unit 1 isshown. The interrogation unit 1 includes a transceiver 202 which may bethe aircraft transceiver or may be a transceiver dedicated for use withthe interrogation unit 1. The transceiver 202 utilizes an antenna 201 totransmit and receive over a LOS communication link. The transceiver 202as well as antenna 201 may be of conventional design. Any knownmodulation types and data rates may be used.

To maximize communication range and minimize the probability ofreception by undesirable or unauthorized entities, data for transmissionand data received by the transceiver 202 is processed by DigitalEncryption. Standard (DES) encryption, BCH forward error correctionencoding, bit interleaving, UART formatting and parity checksum bits.Software to accomplish these tasks is partitioned between an applicationprocessor 205 and a personal computer 210. The application processor 205executes a program instruction set and interfaces to the externalinterfaces such as the transceiver 202 PTT (push to talk) control, Modem-1c. 203, RS-232 DUART 204 (Digital Universal Asynchronous ReceiverTransmitter) and to a GPS receiver 208. The processor 205 may beimplemented with a MC68331 Motorola microprocessor or with othercommercially available processors.

SRAM 206 stores temporary data during operation of the interrogationunit 1. Data stored includes such data as identification codes of thePRC-112 survival radio 3, 4 and 5 and set up configurations for theinterrogation unit 1.

Flash PROM 22 stores the executable programs of the interrogationunit 1. PROM 22 can be reloaded with new operation instructions ifnecessary by using the DUART 204. DUART 204 supports an RS-232 interfaceport. DUART 204 is connected to the application processor 205 andconverts parallel data from applications processor 205 to serial data tomeet the RS-232 standard. The RS-232 interface is used so that thepersonal computer 210 can operate as the human interface to the systemand to permit programming and upgrading the executable software in theapplication processor 205 and the parameters stored in the flash prom22.

Modem 203 creates MSK tones from the digital data stream in the TX Audioinput to the transceiver 202 from the personal computer 210 and alsoconverts received MSK tones on RX Audio output from the transceiver 202to digital data to be sent to the personal computer 210. The MSK modem203 is buffered and level controlled via audio circuitry which providesthe interface to the transceiver 202.

Personal computer 210 provides the user interface to the SAR rescueoperator. The personal computer 210 includes a display (not shown) whichmay be used to display parameters such as identification number,encryption key status, an outgoing message and data received from aPRC-112 survival radio such as identification number, position, time andany message. The personal computer 210 is also used via its keyboard(not shown) to input data for display. The keyboard may provide menuselections of various functions.

The interrogation unit 1 also contains a GPS receiver 208 to provide asource of differential correction data. The GPS receiver 208 also allowsthe relative range between the interrogation 1 and the survival radiounit 3, 4 or 5 to be calculated. The personal computer 210 is used bythe operator to select a message to be sent, to initiate aninterrogation. Personal computer 210 includes a display device topresent information to the operator of the SAR interrogator unit such asthe location of the survival radio unit 3, 4 or 5, messages sent anddistance to the survival radio unit 3, 4 or 5 and to setup systemparameters such as unit identification numbers, encryption keys, whetherthe system is operating in a relay mode and other parameters. The GPSreceiver 208 of the interrogation unit 1 may be of conventional design.

Turning now to FIG. 3, a survival radio unit 3 is shown in blockdiagram. The survival radio unit 3 as noted above can function inseveral modes. In one operational mode, the radio unit 3 will functionas a transponder, detecting a transmitted ID code (DME ID) from theinterrogation unit 1. If the transmitted ID code is identical to thecode programmed into the radio memory (Survivor ID Code), survival radio3 responds by keying a transmitter and transmitting ranging modulation.The survival radio 3 may also operate in an emergency beacon mode. Inthis mode, the survival radio 3 transmits an emergency swept tone beaconsigned compatible with conventional UHF/VHF Automatic Direction Finding(ADF) equipment. In a third mode, the survival radio unit 3 may functionas a conventional two-way radio by providing for voice transmission overLOS paths.

The survival radio unit 3 includes a transceiver 300 coupled to anantenna 301. The transceiver 300 and antenna 301 may be of conventionaldesign. The transceiver 300 includes a PTT control input as well as atransmit audio input, TX Audio, and a receive audio output, RX Output.An application processor 310 is coupled to the transceiver 300. Theapplication processor 310 includes a microprocessor 305 which may, forexample, be a Motorola MC68331 microprocessor in conjunction with aflash EEPROM 306 and a SRAM 307 both of which are commercially availableproducts. The application processor 310 executes a program instructionset and interfaces to a keypad 310, display 309, the PTT control, TXAudio and RX Audio connections to the transceiver 300, the GPS receiver320 and to a programming port 311.

Display 309 may be a LCD (Liquid Crystal Display) device and displaysvarious information such as GPS information, outgoing messageinformation and data received.

Keypad 310 is a part of the user/radio interface and is a multi-buttonkeypad which provides input data as mentioned above for control of theradio 3.

SRAM 20 stores temporary data during operation of the radio 3. Datastored includes such data as GPS data, data received from aninterrogator 1 and various menu selections.

Flash EPROM 306 stores executable programs of the radio 3 as well as theidentification code of the radio 3. EPROM 306 can be reloaded with newprogramming data by using the DUART 304 and the programming port 311.

Buffer 308 is provided as a buffer between the DUART 304 and themicroprocessor 305. Buffer 308 is of conventional design and may beincorporated into the DUART 304.

DUART 304 provides for a RS-232 interface at port 311. The DUART isconnected to the microprocessor 305 via the buffer 308 and convertsparallel data for microprocessor 305 into serial data for the RS-232standard. The RS-232 interface is used primarily for programming andupgrading the executable software in the radio 3.

Digital modem 318 creates the MSK tones from the digital data streamfrom the microprocessor 305 to the TX Audio input of the transceiver 300and also converts the received MSK tones at the RX Audio output of thetransceiver 300 to the digital signals for use by the microprocessor305.

Power supply 319 supplies the particular voltages required for operationof each of the various units included in the radio 3. The power supply319 includes a battery which is not shown.

The radio 3 further includes a GPS receiver 320. The GPS receiver is ofa conventional design and provides the location information of the radio3 and GPS based time. With location information from the GPS receiver320, the microprocessor 305 can provide position and time information onthe display 309 as well as responding to an interrogation request totransmit the positional information to the interrogation unit 1.

In operation, and with reference to FIGS. 1, 2 and 3 when searching fora downed pilot, the interrogation unit 1 formats a survival radiomessage through personal computer 210 and sends the message throughmodem 203 to the transceiver 202 via the TX Audio input to thetransceiver 202. Interrogation is typically made for a specific survivalradio, for example, survival radio 3 by selection of an appropriateidentification number.

When the interrogation unit 1 receives a response from survival radio 3,a message including and identification number, position of the survivalradio 3, the interrogation unit 1 can quickly determine the range andbearing of the survival radio 3 once the position of the helicopter 2 isknown. The downed pilot may then be recovered or his position may bepassed onto another recovery craft or team.

To assure integrity of the messages sent by interrogation unit 1 toradio 3 and from radio 3 to interrogation unit 1, two methods for dataintegrity are employed, i.e., bit interleaving and forward errorcorrection.

Bit interleaving is used to distribute errors that are introduced due tobursts of noise interference that exist for durations of more than onebit.

Forward error correction is used to detect and correct random errors.Numerous classes of random error correcting codes exist, but a preferredone which is extensive and powerful is the Bose, Chauduri andHocquenghem or BCH class code. This code adds additional bits to a blockof data bits and permits both detection and correction of bit errors.

When a survival radio 3 receives a message from an interrogation unit 1,the application processor 310 checks for an ID match and then proceeds.If the ID code matches, the application processor 310 will respond tothe message. If the message includes a position request, the applicationprocessor 310 obtains the latest position of the survival radio 3 fromthe GPS receiver 320, formats and encodes the data in the same manner asdescribed above and provides the information to the transceiver 300which transmits the information to the interrogation unit 1 over the LOSlink 101. If the ID code does not match, the survival radio unit 3 willnot respond.

When an identification code match occurs, if a message was alsotransmitted by the interrogator, the microprocessor 305 will decode andverify the message and the message will be displayed on the LCD display309.

The survival radio unit 3 can also initiate a transmission to theinterrogator 1, such as to send a text message. The user can select astored message or input a free format text message of up to 40alphanumeric characters with the keypad 310 and display 309 on thesurvival radio 3, 4 or 5.

The keypad 310 is used to access a menu shown on the display. The usercan scroll through the available messages which are displayed in text.When the user sees the message that he desires to send, he selects itand the survival radio unit 3 sends the selected message. The keypad 310and display 309 are used to enter alphanumeric pre-formatted messages aswell.

One key of keypad 310 is a send key. The user of radio 3 initiates atransmission by pressing the send key and if the user has enabled thetransceiver 300, the transceiver 300 will transmit the message. If thetransceiver 300 has not been powered up, the unit microprocessor 305cause the display 309 to display an appropriate reminder message. If thetransceiver 300 has been powered up the message will be transmitted tothe interrogator 1 via the LOS path will transmit an appropriatemessage.

The range of the system may be extended to Over The Horizon (OTH} by useof the airborne interrogation platform 2 to "relay" data between thesurvival radio units 3, 4 and 5 and the ground based interrogation unit10. In this arrangement, the airborne platform 2 has LOS paths 101, 102,103 to the survival radio units 3, 4 and 5 and a LOS path 105 to theground based interrogation unit 10. The ground based unit 10 does nothave a LOS path to any of the survival radio units 3, 4 or 5.

In the OTH mode of operation, the ground based interrogation unit 10initiates interrogation of a SAR radio unit 3. The interrogation unit 1located on the helicopter or airborne unit 2 detects the radiotransmission from the ground based unit 10, decodes it and waits apredetermined period of time for the SAR radio unit 3 to respond. If thesurvival radio unit 3 does not respond within the predetermined timeperiod, the airborne interrogation unit 1 re-transmits the interrogationreceived from the ground based equipment 10. The SRAM 206 of theairborne interrogation unit 1 stores the data interrogation transmissionby the ground based unit 10 and re-transmits the interrogation. Becausethe SAR radio unit 3 is within LOS of the interrogation unit 1, itreceives the interrogation transmission and responds accordingly. Theairborne unit 1 detects the reply from the SAR unit 3 and re-transmitsthe reply to the ground based interrogator 10 thereby extending theeffective range of the system as far as the ground based interrogator 10is concerned.

The relay or OTH mode provides a significant advantage for search andrescue. The airborne platform 2 having an interrogation unit 1 operatingin the relay mode may be an unmanned airborne vehicle such as a drone.This allows the interrogation unit 10 to be located in an areacontrolled by friendly forces. The relay unit 1 can be carried on anunmanned vehicle 2 going into the unfriendly area and searching. Thetime that the unmanned vehicle 2 is exposed to unfriendly forces may, ingeneral, be quite large.

With the use of GPS in both the interrogation unit 1 and the survivalradio unit 3 a high degree of accuracy is obtained in a relativedistance between the two units. Either commercial GPS or the militaryGPS may be used and both provide a high degree of accuracy of less than5 meters deviation from the actual distance. This results from the factthat the absolute position of the survival radio unit 3, 4 or 5 is notof as much significance as is the relative locations of the survivalunit and the interrogation unit 1. Because the units are in closeproximity, within a couple hundred miles of each other, they have thesame error in absolute position. Even though the absolute position couldbe off by as much as several hundred meters, the relative positionbetween the two units is precise, because they both have the sameerrors, resulting from their being in the same geographical area. Usingthe same satellite constellation, the system will have a relativeaccuracy equivalent to that of precise position system GPS like adifferential GPS. In addition, differential correction factors may betransmitted in the data stream.

Each survival radio unit 3, 4 and 5 has 255 "canned" messages. Inaddition, each survival radio unit 3, 4 and 5 includes the capability tosend a free format 40-character alphanumeric test message entered viathe keyboard on the radio. The message composed and sent can be in anylanguage and the operator of the unit 2, does not have to vocalize amessage and risk being heard.

The messaging structure is set up to keep transmission time to a minimumto reduce the likelihood that a downed pilot could be located bydirection finding on his transmitted signal. One objective is to keepair time of any reply to a minimum.

The messaging protocol utilized includes what are called "shortpackets". For longer messaging structures like the canned messages, morepackets are added to the message.

Burst data transmission is utilized in all transmissions to and from thesurvival radio units 3, 4 and 5 rather than a continuous data stream, toreduce the chance of reception by unauthorized or undesirable elements.To extend the range of this system, interleaving and forward errorcorrection corrects for errors in the burst transmission.

The data format for interrogation unit and survival radio unit responsemessages is shown in FIGS. 4, 5 and 6. Four message formats are utilizedin a system in accordance with the invention. The four types of radiomessages are Short Uplink, Long Uplink, Short Downlink and LongDownlink. As shown in FIG. 4, all messages are transmitted in packetsand all share a common header which is sent in a standard packet 41. Theremainder of the message comprises short packets 42.

The standard packet is shown in FIG. 5 and is 536 bits in length. Theformat includes 16 bits as the Bit Synch 51, 16 bits for a Unique Word52, 56 bits as a header 53, 256 bits of encrypted "Radio Message Data"54 and the remaining 152 bits are used for forward error correction(FEC) BCM code 55 and pad bits 56.

The "Unique Word" packet 52 which is two bytes or 16 bits represents aunique word to identify the start of a message. The "Dest" block 57comprises 16 bits representing the user ID destination that is toreceive the data. The "Source" block 50 comprises 16 bits representingthe user ID source that sent the data. The "Type" block 59 comprises 8bits used to identify one of four radio message protocols as follows:

    ______________________________________                                        Description    Bit Representation                                                                         Value                                             ______________________________________                                        Short uplink   0011 0011    51                                                Long uplink    0011 0100    52                                                Short downlink 0011 0101    53                                                Long downlink  0011 0110    54                                                ______________________________________                                    

The "Seq" block 59a comprises 8 bits representing the number of shortpackets following the standard packet. "Flags" block 59b comprises an 8bit field used to identify 3 options: Encryption, Relay, and DelayResponse (2 bits unused).

    ______________________________________                                        Description    Bit Representation                                             ______________________________________                                        Encrypted Data XXXX XXX1                                                      Relay          XXXX XX1X                                                      Delay Response XX11 11XX                                                      ______________________________________                                    

Short packets as shown in FIG. 6 are 192 bits in length and provide 128bits for the radio message data 61. The remaining 64 bits are used forFEC BCM data 62 and pad bits 63. As seen in FIG. 6, part of the standardpacket uses FEC. Only the radio message data 61 is encrypted. The entireshort packet uses FEC. The standard packet header data "Bit Sync" 51comprises 16 bits representing alternating O's and 1's.

Turning now to FIG. 7, the "Short Uplink" format is shown. An example ofthis message is a "send" with a canned message. This message reports thedestination ID and source ID, position data, time fix, send time, datum,message number, purge flag, and delay response. The message comprises32-8 bytes arranged as shown.

FIG. 8 illustrates message ID for a "Long Uplink." An example of thismessage is a "send" with a free-formatted text message. This messagereports the destination ID and source ID, position data, time fix, sendtime, datum, message number, purge flag, delay response, andfree-formatted text message.

FIG. 9 illustrates a message ID for a "Short Downlink." An example ofthis message is a "ping" with a canned message. This message reports thedestination ID and source ID, message number, purge flag, and delayresponse. The text message number identifies a "canned message" beingsent.

FIG. 10 illustrates a message ID for a "Long Downlink." An example ofthis message is a "ping" with a free-formatted text message. Thismessage reports the destination ID and source ID, message number, purgeflag, delay response, and free-formatted message.

Transmission and reception of data is accomplished using minimum phaseshift keying, (MPSK), format at nominal baud rate of 1200 or 2400. Thisbaud rate allows the data to be transmitted over a radio with abandwidth that is compatible with an existing half-duplex voice channel.While the present implementation uses AM modulation, the technique isequally applicable to FM or PM modulation techniques.

A TDMA/CDMA message structure protocol configuration is used to assurethat "collisions" do not occur due to high messaging traffic. Thistraffic density is increased by such functions as relay operation inwhich one or more intermediate fixed or moving sites are involved andtext/canned messaging is used.

Message performance/reliability is enhanced by use of severaltechniques. A cumulative checksum is performed on the raw data used ineach message. The raw data is encrypted using DES type 3 encryptiontechniques. The data may be further enhanced by the use of a ForwardError Correcting, FEC, code such as the Bose-Chadhuri-Hocquenghem, BCH,code which has the capability of not only detecting bit errors but alsocorrecting multiple bit errors, for example, a 45/63 code length willcorrect up to three errors. Bit interleaving is used which distributes abyte across the entire message and makes the data much less susceptibleto bursts of bit errors which may occur due to aperiodic bursts ofinterference.

Although the invention has been described in terms of a preferredembodiment, it will be readily apparent to those skilled in the art thatvarious modifications may be made without departing from the spirit orscope of the invention.

What is claimed is:
 1. A Search and Rescue System comprising:at leastone survival radio, said survival radio comprising a GPS module forobtaining GPS derived location information and a transceiver forproviding Line of Sight (LOS) communications to transmit said locationinformation in response to receipt of an interrogation signals; a firstinterrogation unit, said first interrogation unit having apparatus forsending interrogation signals to said survival radio to cause saidsurvival radio to transmit said GPS derived location informationsignals, said interrogation signals being sent via a substantially Lineof Sight (LOS) first communication link to said survival radio, said GPSderived location information signals being received from said survivalradio over said first communication link, wherein said firstinterrogation unit is carried on an airborne vehicle; at least onesecond interrogation unit, said second interrogation unit havingapparatus for sending second interrogation signals to said firstinterrogation unit via a substantially LOS second communication link,said second communication link being between said first interrogationunit and said second interrogation unit, said first and secondinterrogation units being cooperatively operable such that said firstinterrogation unit relays information between said second interrogationunit and said survival radio such that substantially Over the Horizon(OTH) communication is obtainable between said survival radio and saidsecond interrogation unit over said first and second communicationlinks, at least part of said information comprising GPS derived locationinformation identifying the location of said survival radio.
 2. A searchand Rescue System in accordance with claim 1, wherein:said firstinterrogation unit is carried on an unmanned airborne vehicle.
 3. Searchand Rescue System in accordance with claim 1, wherein:said secondinterrogation unit is at a ground based location, said secondinterrogation unit including apparatus to provide GPS correctioninformation to said first interrogation unit.
 4. A Search and RescueSystem in accordance with claim 3, wherein: said first interrogationunit transmits said GPS correction information to said survival radio.5. A Search and Rescue System in accordance with claim 1, wherein:saidat least one survival radio includes means for selecting one of aplurality of stored messages for delivery to said first interrogationunit.
 6. A Search and Rescue System in accordance with claim 5, wherein:said survival radio includes in each message transmitted anidentification code to uniquely identify said survival radio.
 7. ASearch and Rescue System, comprising:a survival radio, said survivalradio comprising a GPS receiver, a transceiver, and a controller, saidsurvival radio having a unique identification code associated therewith,said survival radio controller responding to interrogation messagesreceived over a substantially line of sight first communication path bysaid transceiver and having contained therein said unique identificationcode by causing said transceiver to transmit location informationgenerated by said GPS receiver and said identification code over saidfirst communication path; a first interrogation unit comprising a secondcontroller and a second transceiver, said second controller beingoperable to cause said second transceiver to transmit interrogationmessages containing said survival radio identification code, said secondcontroller being further operable to identify messages received by saidsecond transceiver as being from said survival radio; and a secondinterrogation unit comprising a third controller and a thirdtransceiver, said third controller being operable to cause said thirdtransceiver to generate said interrogation messages and to transmit saidinterrogation messages to said first interrogation unit, said firstinterrogation unit being responsive to said interrogation messages tore-transmit said interrogation messages to said survival radio, at leastsome of said interrogation messages including GPS position correctiondata that can be used by said survival radio to correct a local positiondetermined by said GPS receiver, said at least some interrogationmessages being among said interrogation messages that are re-transmittedto said survival radio to cause said survival radio to transmit saidlocation information over said first communication path.
 8. A Search andRescue System in accordance with claim 7, comprising:a substantiallyLine of Sight (LOS) first communication path between said survival radioand said first interrogation unit; and a substantially LOS secondcommunication path between said first interrogation unit and said secondinterrogation unit.
 9. A Search and Rescue System in accordance withclaim 8, wherein:said first communication path and said secondcommunication path are operable to provide an Over the Horizon (OTH)communication path between said survival radio and said secondinterrogation unit.
 10. A Search and Rescue System in accordance withclaim 7, comprising:a third GPS receiver in said second interrogationunit; where in said third controller generates said GPS correction datafrom GPS information obtained from said third GPS receiver and datapertaining to the geographic location of said second interrogation unit.11. A Search and Rescue System in accordance with claim 7, wherein:saidGPS correction data includes GPS differential correction data.
 12. Amethod of operating a Search and Rescue system, said system comprisingat least one survival radio, at least one first interrogation unit, anda second interrogation unit, said method comprising the stepsof:generating an interrogation message at said second interrogationunit, said interrogation message containing a unique identification codecorresponding to a first survival radio; transmitting said interrogationmessage to said first interrogation unit, wherein said firstinterrogation unit is carried on an airborne vehicle; utilizing saidfirst interrogation unit to relay said interrogation message to saidfirst survival radio; responding to said interrogation message, at saidfirst survival radio, by generating and transmitting a response messagecontaining said unique identification code and geographic locationinformation; receiving said response message at said first interrogationunit; and relaying said response message from said first interrogationunit to said second interrogation unit.
 13. A method in accordance withclaim 12, comprising:receiving at said survival radio GPS signals andgenerating said geographic location information from said GPS signals.14. A method in accordance with claim 13, comprising:generating at saidsecond interrogation unit GPS correction information and transmittingsaid GPS correction information to said survival radio via said firstinterrogation unit.
 15. A method in accordance with claim 14,comprising:receiving GPS signals at said second interrogation unit anddetermining said GPS correction information from said GPS signals anddata pertaining to the geographic location of said second interrogationunit.
 16. A method in accordance with claim 12, wherein:said step ofgenerating an interrogation message includes encoding said interrogationmessage using an error correction code.
 17. A method in accordance withclaim 12, wherein:said step of generating an interrogation messageincludes utilizing bit interleaving.
 18. A method in accordance withclaim 15, comprising:receiving GPS signals at said first interrogationunit and determining the position of said first interrogation unit fromsaid GPS signals.
 19. A method in accordance with claim 14,comprising:using said GPS correction information at said firstinterrogation unit to determine the position of said first interrogationunit.
 20. A method in accordance with claim 15, comprising: operatingsaid second interrogation unit at a fixed ground based location duringthe duration of a search.